1. Field of the Invention
This invention relates to apparatus and method for protecting the core of a pressurized water reactor from departure from nucleate boiling and hot leg boiling. More particularly, it is directed to such an apparatus and method which trips the reactor if operating limits set by a segmented delta temperature set point boundary are exceeded.
2. Background Information
Fuel integrity is an important safety and economic concern in nuclear power plants. In recognition of this, it is known to employ an overpower delta temperature reactor trip function to protect against fuel melting and an over temperature delta temperature reactor trip function to protect against departure from nucleate boiling and hot leg boiling. If departure from nucleate boiling is precluded, adequate heat transfer is assured between the fuel rod cladding and the reactor coolant, and therefore potential damage due to inadequate cooling is prevented. The hot leg temperature must be less than the saturation temperature. The protection system uses the reactor vessel temperature difference (delta temperature) as a measure of core power. To assure that the delta temperature is proportional to core power, hot leg boiling must be precluded.
The limits imposed upon plant operation by these phenomenon can be presented on a plot of reactor vessel delta temperature (.DELTA.T) versus reactor coolant system average temperature (T avg) as shown in FIG. 1 where line 1 is the departure from nucleate boiling core limit line and line 3 is the hot leg boiling limit line. As can be seen, these core limits define line segments of different slope which intersect. An overtemperature .DELTA.T set point 5 has been developed to prevent the core from reaching the core limit lines 1 and 3 by tripping the reactor before the measured vessel delta temperature reaches either line of the core limits. Graphically, the core operating point 7 must remain to the left of the core limit lines 1 and 3 in FIG. 1. Current set point methodology generates an overtemperature .DELTA.T set point 5 which is a straight line, even though the core limits are made up of two line segments 1 and 3. The single line segment overtemperature .DELTA.T set point 5 results in a significant loss of operating margin between this set point line and the full power operating point of the reactor. This margin is currently not available to the reactor operator. Over the past several years, accident analysis margins associated with the departure from nucleate boiling related events have continually eroded. Contributing factors to the situation are advanced fuel features, higher fuel peaking factors which are required to accommodate longer fuel cycles, plant upratings, and reactor coolant system flow reductions. Furthermore, although not usually as severe, hot leg boiling margins are impacted by reactor coolant system flow reductions, upratings, or similar plant changes. As these margins are eroded, the corresponding core thermal limit lines become more limiting. This in turn results in more limiting overtemperature .DELTA.T set points.
More limiting overtemperature .DELTA.T set points can impede plant operation by increasing the potential for plant trip. The ability of the plant to "ride out" an operational transient such as a partial loss of feed water or load rejection is reduced by restrictive overtemperature .DELTA.T set points. Furthermore, restrictive set points may result in partial trip signals in the multichannel protection system should the process parameter signals feeding into the set point algorithm be somewhat noisy. This places the plant in a partial trip situation which could lead to an actual reactor trip should the plant experience a minor transient leading to any one remaining channel tripping.
A limiting factor to providing less restrictive overtemperature .DELTA.T set points is the space available in the cabinets for the protection system which requires redundancy for reliability.
There is need, therefore, for an improved method and apparatus for protecting a pressurized water reactor from departure from nucleate boiling and hot leg boiling.
There is also a need for such an improved method and apparatus which improves the operating margin available to the operator.
There is a further need for such a method and apparatus which reduces the potential for unnecessary tripping of the reactor.
There is an associated need for such an improved method and apparatus which can be implemented in the available protection system cabinet space.